In recent years, there has been rapidly expanded in use of a flat panel display (FPD) device such as a liquid crystal display device or an organic EL (electroluminescence) display device. Among them, an active matrix display device is exemplified as a flat panel display device which can carry out a high grade display.
The active matrix display device adopts amorphous silicon TFTs (Thin-Film Transistors) or polysilicon TFTs (Thin-Film Transistors). Such amorphous silicon TFTs (Thin-Film Transistors) and polysilicon TFTs have optical sensitivity in a visible region. Therefore, carriers are generated while the amorphous silicon TFTs and the polysilicon TFTs are irradiated by light. This causes a reduction in resistance of such TFTs, thereby giving rise to a deterioration in displaying quality of the active matrix display device. In order to prevent such a deterioration in displaying quality, the active matrix display device adopts an arrangement in which a light-blocking layer (e.g. metal coated layer) for blocking incident light is included. This prevents a change in resistance. However, since the arrangement causes a reduction in effective displaying area, it is necessary for a backlight to emit brighter light. This brings about a new problem of a deterioration in efficiency of energy use. In view of the circumstances, developments of a transparent transistor have been promoted which does not have optical sensitivity in the visible region and can be used as a transistor for use in a device such as the active matrix display device.
Such a transistor adopts a transparent active layer which is made of a material such as zinc oxide (ZnO), zinc magnesium oxide (MgXZn1-XO), zinc cadmium oxide (CdXZn1-XO), or cadmium oxide (CdO). Especially, a transistor attracts attention whose active layer is made of zinc oxide since the zinc oxide is a semiconductor which shows a relatively good physical property during a manufacturing step at a low temperature. However, a transistor whose active layer is made of zinc oxide alone would not have a transistor characteristic sufficient to come in practice. In view of this, a technique has been developed to improve a transistor characteristic, by adding impurities to a zinc oxide film. For example, Patent Literatures 1 through 3 disclose this kind of technique.
The Patent Literature 1 teaches that a transistor is made transparent by using (i) a channel layer made of a transparent semiconductor such as ZnO and (ii) a gate insulating layer made of a transparent insulating material such as insulating ZnO to which a univalent or a pentavalent element is added.
The Patent Literature 2 teaches that a channel layer can have a high resistance by adding a 3d transition metal element (e.g., Ni) to a transparent channel material (e.g., ZnO) and it is possible to realize a high-efficiency thin-film transistor which has a good on-off ratio and mobility even if a thin film is formed at a relatively low temperature (e.g., at room temperature).
The Patent Literature 3 discloses a method for manufacturing an active layer of a thin-film transistor element, in which (i) an active layer is formed with the use of a ZnO precursor solution, to which a dopant is added in advance, by means of a chemical bath deposition method, a photochemical deposition method, or a sol-gel process, and (ii) the dopant is concurrently added to the active layer. The Patent Literature 3 teaches that the method makes it possible to realize a thin-film transistor which can be operated under high pressure and has a good element characteristic. In addition, a valence VIA element such as S is disclosed as the dopant.
Further, there is another problem that a transistor whose active layer is made of a polycrystalline film may have a deteriorated transistor characteristic due to grain boundary of the polycrystalline. This necessitates suppression of an influence of such a grain boundary on a transistor characteristic. In view of this, for example, a polycrystalline silicon transistor which comes in practice as a transistor whose active layer is made of a polycrystalline film employs an SOI technique in which a polycrystalline or amorphous silicon in the active layer is irradiated by a laser beam so as to be transformed into a monocrystalline silicon.